1. Field of the Invention
The present invention generally relates to lifting heavy equipment and, more particularly, is concerned with apparatus for engaging heavy equipment including blow out preventors and diverters used on drilling rigs in such a way that the drawworks of a drilling rig can be utilized in the lifting process.
2. Description of the Prior Art
The development of the present invention comes in response to the difficulties encountered when lifting diverters--devices which often weigh somewhere in the range of 50,000 lbs. Diverters are pressure control devices used frequently on offshore floating platforms. The modern versions of the diverter may best be described as a low pressure blow out preventer (B.O.P.).
Floating platforms have their main B.O.P. stacks located on the sea bed. Telescoping risers reach from the B.O.P.'s on the seabed to the floating platform. The diverter is mounted on the top of the telescoping risers.
Due to the heavy weight of the diverter, it is necessary to use the blocks of the rig to pick it up. Therefore, a crossover tool which connects the diverter to the blocks is required. In the trade, this type of tool is referred to as a diverter handling tool or a diverter running tool.
The running tool must be capable of engaging the diverter in such a way that the diverter will not be damaged and so that personnel will be protected from accidents. In some conceivable situations, the running tool may be called upon to lift the entire assembly of diverter, risers, and B.O.P. stack. The weight of this assembly can run up to 750,000 lbs. Therefore, in designing a running tool and including safety factors, the pulling strength of the running tool may be in the 2,000,000 lb. range.
The presently available diverter tools have a number of shortcomings.
A design problem arises from the massiveness of the running tools because it is difficult to design an effective engaging mechanism that is also straightforward for the rig crew to operate. Representative instructions for engaging the presently available running tool to the diverter include a right and left turn of the inner with respect to the outer body of the tool interspersed with a requirement to lift up on the tool. In the course of these instructions, it becomes difficult to maintain vertical alignment of the dogs, or engaging elements, to the groove in the diverter bore in which they must engage.
To overcome such problems, presently available running tools employ dogs that have engaging surfaces angled to about 45 degrees. In this way, the dogs can pull the tool into correct alignment as engagement force is applied if the tool is roughly at the correct position. However, this solution creates other problems.
When vertical lifting forces are applied to 45 degree angled engaging surfaces, an inwardly directed horizontal or radial resultant force is produced that is equivalent to the vertical lifting force. This radial force pushes inwardly on the engaging surfaces and creates a tendency for the tool to disengage. Also the radial force causes wear on the inner body of the tool that is not readily detectible and that may result in a loose fit of the running tool with the diverter.
The prior art tools have no positive means for retracting the dogs. Instead, the inner body is moved so that a cavity exists into which the dogs should retract. When the tool is lifted, the same radial force that biases the dogs to disengage as discussed above, also forces the dogs to retract into the cavity. This procedure creates the possibility of sticking and jerking when attempting to disengage the dogs. Also, if the dogs are not completely retracted, it's possible that jerking and dragging may damage some of the sealing elements of the diverter as the tool is removed from the bore of the diverter.
With regard to the sealing elements, the presently available tool does not allow for pressure testing of the diverter. So once the diverter is installed, the running tool must be removed prior to pressure testing the diverter. If the diverter does not seal and the diverter must be removed for repair, then the running tool must be reinstalled. This wastes time. As well, some diverters require up to four hours before the sealing elements become relaxed enough so that the running tool can be reinstalled. Considering the premium that is placed on offshore rig time, this could be a costly shortcoming.
Another problem that may develop with these tools is that if the tool is improperly aligned or if there is a failure in the engaging mechanism, and a large engaging force is applied, the tool may be jammed into a binding, or cocked, position that will take some time to correct.
Other means of lifting the diverters have been considered. For instance, rather than attempting to engage the running tool in the bore of the diverter below the sealing mechanisms, it's been suggested that a housing be mounted on top of the diverter. A more simple engaging means, such as a J - slot locking device could be used inside such a housing. However, companies which utilize the diverters feel that using the space above the diverter for a housing is not feasible. The simple J - slot locking device is also not feasible for use within the bore of the diverter below the sealing mechanisms because grooves would have to go through the sealing area of the diverter.
Consequently, a need exists for improvements in lifting techniques of heavy equipment such as diverters which will result in running tools that are easier and safer for the rig crew to operate.